1. Field of the Invention
The present invention relates to a laser power sensor for measuring an intensity of a laser beam.
2. Description of the Related Art
In a near-infrared laser device such as a carbon dioxide laser, a power sensor for measuring an actual laser power is often used, in order to control the laser power. For example, a thermoelectric power sensor is configured to receive monitor light of from several watts to several tens of watts extracted from a rear mirror of a resonator, and measure laser power by converting the received light to a voltage signal. Such a power sensor has a substrate (or a thermal disc) constituted from a high thermal conductive material, and a thermocouple positioned around a portion of the substrate (or a light-receiving portion) by which the monitor light is directly received, wherein a voltage is generated due to a difference between the temperature of the light-receiving portion and the ambient temperature.
When the above power sensor is used for a high-power laser device, the temperature of the substrate (thermal disc) is excessively increased as the intensity of the monitor light from the rear mirror is increased. In such a case, the function of the power sensor may be deteriorated as a result of damage to the light-receiving portion or the thermocouple. Therefore, it is necessary to design the power sensor based on the intensity of the monitor light from the rear mirror.
As a relevant prior art document, JP 2005-091271 A discloses a laser power monitor including: a light-receiving part for converting energy of an incident laser beam to heat; a heat-releasing part adjacent to the light-receiving part and for releasing the heat converted by the light-receiving part; and a temperature detecting part for measuring the temperature of the light-receiving part, wherein a measuring range of the temperature detecting part is wider than a beam diameter of the laser beam of a portion of the light-receiving part which does not contact the heat-releasing part, whereby the temperature can be measured in a non-contact manner.
JP 2010-212552 A discloses a laser oscillator including a diffuse-absorber for darkening a laser beam, a laser power detector for detecting the darkened laser beam, and a chamber-type limiting means for limiting an amount of incident light toward the laser power detector, whereby measurement accuracy of the laser power detector may be improved and the laser power can be stably controlled.
JP H07-181078 A discloses an attenuator for a laser power sensor, including: an introducing part for introducing a laser beam; an attenuating part having a cylindrical member connected to the introducing part and having a reflective surface formed on an inner surface of the cylindrical member for reflecting the laser beam; and a target plate having multiple pyramids or cones in a grid pattern with a pitch smaller than a diameter of the laser beam, the target plate being adhered to the attenuator so that the irradiated laser beam is divided by being reflected by surfaces of the pyramids or the cones and directed toward the inner surface.
Further, JP 2677845 B discloses a laser power measurement device including an integrating sphere having multiple holes formed on an incident part thereof so as to transmit a laser beam, and an output detecting sensor arranged at an appropriate portion of the integrating sphere for detecting an output of the laser beam.
In the prior art, in order to prevent a light-receiving part, etc., from being damaged by heat, the following two general options may be used.
(1) Depending on an increase in the intensity of a laser beam, a heat capacity of the light-receiving part (thermal disc) is increased (for example, the plate thickness or the diameter is increased), or a cooling capacity of the light-receiving part is increased, so as to limit an increase in the temperature of the light-receiving part.
(2) A laser absorber such as a filter or an integrating sphere is positioned in front of the light-receiving part, so as to limit the intensity of the laser beam toward the light-receiving part.
Regarding option (1), when the heat capacity of the light-receiving part is increased, a temperature rise speed thereof is decreased, and thus a response speed of an output voltage of the thermocouple relative to the incident laser beam, which is an important parameter of the power sensor, may be decreased. On the other hand, when the cooling capacity of the light-receiving part is improved by forced-air-cooling or water-cooling, the size and cost of the power sensor are inevitably increased. In addition, it is difficult to control the improved cooling capacity, and therefore, when the cooling capacity is not appropriately controlled, the temperature measurement by the thermocouple may be affected, whereby the laser power may not be correctly measured and may not be appropriately controlled.
Regarding option (2), as a method or means for decreasing the intensity of the incident laser beam, a half mirror or an ND (neutral density) filter may be used. However, such means is a precise and expensive optical component, and it is necessary to carry out maintenance (e.g., cleaning and replacement) in order to ensure the reliability of the power sensor for a long time. Accordingly, the cost of the power sensor is significantly increased. Further, the means for reducing the intensity of the laser beam by reflection or diffusion, such as an integrating sphere, includes many components and is expensive, and requires a large location space, and thus the laser oscillator may be enlarged.